1. Field of the Invention
The present invention is related to an optical beam scanning apparatus, and more particularly to the optical beam scanning apparatus and a digital writing apparatus, which are mounted in an image forming apparatus implementing an optical beam scan method and perform image writing.
2. Description of the Related Art
An image forming apparatus such as a digital composition apparatus and a laser printer for conducting an image form in an optical beam scan method includes an optical beam scanning apparatus for controlling a laser diode to turn on and for deflecting an optical beam emitted from the laser diode by using a polygon mirror which is rotating to scan in the main scan direction, in order to write an image for each of main scan lines on a photosensitive drum.
In the above-described optical beam scanning apparatus, for example, as disclosed in Japanese Laid-open Patent Application No. 2003-29181, in order to make an image writing position be constant on the photosensitive drum for each of the main scan lines, a sensor (hereinafter, referred as a synchronization detection sensor) is generally mounted to detect an optical beam outside an image writing area at an image writing start side. The optical beam being scanned by the polygon mirror is detected by the sensor, and a lighting timing of the laser diode is regulated by using a synchronization detection signal, which is generated, as a reference.
FIG. 1 is a diagram illustrating a schematic configuration of a conventional optical beam scanning apparatus 200. The optical beam scanning apparatus 200 illustrated in FIG. 1 includes laser diodes 202Y, 202M, 202C, and 202K, a polygon mirror 204 for deflecting the optical beam emitted from the laser diodes 202Y through 202K, scan lenses (fθ lens) 206a and 206b for making the optical beam being deflected by the polygon mirror 204 scan on a subject scan surface at the same speed, a first mirrors 212a and 212b respectively forming a reflecting mirror group, synchronization detection sensors 210a and 210b respectively for detecting the optical beam, and mirrors 208a and 208b respectively for reflecting the optical beam being outside a direction toward the subject scan surface to direct the optical beam to the synchronization detection sensors 210a and 210b. 
The optical beam scanning apparatus 200 illustrated in FIG. 1 is configured as an optical beam scanning apparatus of a bidirectional scan type. The laser diodes 202C through 202K are implemented for Cyan (C), Magenta (M), Yellow (Y), and Black (K), respectively. In order to deflect a beam light for each color, an upper portion and a lower portion are separately used in each reflection surface of the polygon mirror 204. The laser diodes 202K and 202Y for black and yellow emit optical beams to the lower portion of the reflection surface of the polygon mirror 204 and the laser diodes 202C and 202M for cyan and magenta emit optical beams to the upper portion of the reflection surface of the polygon mirror 204. The optical beams emitted from the laser diodes 202Y and 202M are reflected on the reflection surface of the polygon mirror 204, pass the scan lens 206b, are reflected at the first mirror 212b, and are scanned on corresponding photosensitive drums by a rotation of the polygon mirror 204. Also, the optical beams enter the synchronization detection sensor 210b mounted at an edge of the writing start side in a main scan direction. When detecting an incidence light of the optical beam, the synchronization detection sensor 210b outputs a synchronization detection signal which is a reference of controlling a lighting of a corresponding laser diode. In the same manner, optical beams emitted from the laser diodes 202K and 202C for black and cyan pass the scan lens 206a, are reflected at the first mirror 212a, and are scanned on corresponding photosensitive drums. When detecting an incidence light of the optical beam, the synchronization detection sensor 210a outputs a synchronization detection signal.
However, in the above-described conventional configuration, since the synchronization detection sensors 210a and 210b are used for the lighting control of the laser diodes 202C through 202K, spaces for mounting the synchronization detection sensors 210C and 210K and spaces for leading the optical beams to the corresponding synchronization detection sensors 210a and 210b are required. Also, the synchronization detection sensors 210a and 210b and optical elements are costly. In addition, the conventional configuration is not suitable for reducing the spaces and costs.